1. Field of the Invention
This invention relates to the production of elastomeric polyurethane compositions. More specifically it relates to polyurethanes of improved resistance to flex cracking and cut growth prepared by the addition of a controlled amount of a polytetra-hydrofuran diol of a specific molecular weight range to a prepolymer comprising a polyether diisocyanate followed by reaction with a diamine chain extender.
2. Related Prior Art
Numerous attempts have been made to improve the resistance of polyurethane elastomers to breaking upon flexing and bending and to resistance to cut growth. However improvement in such properties is generally accompanied by losses in other properties such as tensile and modulus strengths.
U.S. Pat. No. 3,798,200 alleges the improvement of cut growth and flex cracking resistance by preparing a polyurethane elastomer by reacting a diisocyanate, such as toluene diisocyanate, with a polyether glycol having an average molecular weight of 4,000 to 20,000 and having at least two peaks in the molecular weight distribution curve, at least one of which peaks is located in the higher molecular weight region and at least another of which peaks is located in the lower molecular weight region of the polyether.
The same patentees obtained a later patent, U.S. Pat. No. 3,963,681 claiming improvement in the cut growth and flex crack resistance by preparing a polyurethane from a polyether glycol having a weight average molecular weight in the range of 1,000 to 4,500 and having a molecular weight distribution curve with at least two peaks, at least one peak being in the lower part of this range and at least one peak being in the higher part of this range.
While the molecular weight distribution is described as given above, the various examples show that the polyether glycols comprise a typical mixture, for example, one polyether glycol having a weight molecular weight 8,500-11,000 but the proportions of all the glycols present are such that the average molecular weight of the mixture as a whole is below 4,500.
The patentees describe their preparation process as being either (a) a one-stage reaction which comprises the polyether diol or polyether glycol reacting simultaneously with the polyfunctional isocyanate and the curing agent, or (b) a two-stage reaction to produce first a prepolymer by reation of the polyfunctional isocyanate with the mixture of polyether glycols and subsequently reacting this prepolymer with the curing agent to produce the cured elastomer.
However still greater improvements in resistance to flex breaking and in cut growth are desirable without sacrifices in other physical properties.